arm64: fix syscall_fn_t type

Syscall wrappers in <asm/syscall_wrapper.h> use const struct pt_regs *
as the argument type. Use const in syscall_fn_t as well to fix indirect
call type mismatches with Control-Flow Integrity checking.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>